NSF Award
2420053
The Florida Current represents the origins of the Gulf Stream that flows northward into the high latitude North Atlantic, eventually becoming the North Atlantic Current. The flow of this water mass warms the atmosphere above it, redistributing heat from the tropics to higher latitudes. As the North Atlantic Current cools near the Arctic, it becomes dense and sinks to the deep ocean then flows southward and in part drives the large-scale Atlantic meridional overturning circulation, which moderates climate in the Northern Hemisphere and beyond. Much attention has been focused on the possible slowdown of this circulation. Although researchers have some understanding of how the Florida Current has behaved in modern times, based on direct measurements, its past behavior is poorly constrained. Further, observations of the Florida Current are limited to recent decades and it has been suggested that the Florida Current has weakened over the last 40 years due to human caused climate change. However, longer-term (multidecadal to centennial) annual resolution data on the Florida Current are currently too scarce to confirm this. Because the period of direct instrumental observation is relatively short, to understand the natural variability of the system, the Florida Current must be studied by natural climate archives and proxy records. This investigation will utilize previously collected corals that are strategically located to address the questions: 1) What is the natural variability in annual changes to the Florida Current over the past several centuries? 2) How do those changes relate to climate drivers in the region, including decadal scale trends to regional and local conditions? The broader impact activities of this proposal include support for several underrepresented researchers, training and mentoring of undergraduate and graduate students, and two postdoctoral researchers. The investigators will create a museum display in the Hall of Planet Earth at the American Museum of Natural History.<br/><br/>Direct observations of the components of the Atlantic Meridional Overturning Circulation (AMOC) are limited to the last couple of decades. Measurements of the Florida Current, a critical component of AMOC, have only existed for the last 40 years with recent work suggesting a modest decline in the Florida Current over the past century; however, annual resolution and long-term (> 50 to 100 years) data on the Florida Current and AMOC are necessary to further evaluate this. The proposed research will contribute much needed information on the rates and processes of the Florida Current over the past 200-300 years. Using previously collected Siderastrea and Colpophyllia corals from Tobago and the Florida Straits that faithfully record oceanographic and climate signals within the geochemistry of their skeletons, this work will reconstruct Florida Current flow from water mass properties. Annual-resolution measurements of radiocarbon content will resolve water mass source variations between several inflow routes into and through the Caribbean Sea that contribute to the velocity of the Florida Current. Sea surface temperature (SST) and salinity (SSS) obtained at monthly resolution from ratios of strontium to calcium (Sr/Ca) and oxygen isotopes (δ18O) respectively will add higher resolution information on water mass properties due to specific hydroclimates in water source regions. The corals are strategically located: one is in the heart of the Florida Current, the second is at the southern end-member of the Caribbean in-flow. Together, these sites will “close the loop” on interactions between the Florida Current and the relative contribution of northern and southern water to the Caribbean current. By integrating data from several locations, this work will better constrain multidecadal variability in the rate of the Florida Current flow before the onset of anthropogenic changes. Box models utilizing the water mass signals of radiocarbon, SST and SSS will be combined with reanalysis of regional Lagrangian output Ocean General Circulation Models to reconstruct decadal resolved changes to the Florida Current source water over the last several centuries.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
